Again and again guards in the mine were killed.
So if we want to go to guarded mine we should go there with full eq and with all gang members to gather some ore and min?!
My suggestion(again! this should be done a long time ago): turn off the FOV for all the guards, it will be a little harder to kill all
or: give them a radio to communicate. If one being shoot then all will go there to help, even if they will not seeing the shooter
You can add there some better guards too